The relationship of plasma somatomedin (IGF-1) to lamb growth rate

Somatomedin (IGF-1) was measured in plasma samples obtained from lambs undergoing SRIF immunization to improve their rate of growth. Weight gains and blood samples were obtained at predetermined intervals between weaning and slaughter for a total of 78 crossbred ewe, ram and wether lambs in three experiments. Plasma IGF-1 concentrations were not altered by SRIF-immunization despite improved rates of gain in wether and ram lambs. Ewe, wether and ram lambs had similar plasma IGF-1 concentrations until 12 wk old. After 12 wk of age, ram lamb IGF-1 plasma concentrations were significantly higher than those of ewe and wether lambs. Fasting lambs for 24 h reduced plasma IGF-1 concentrations, having the greatest effect on heavier lambs. When lambs were allowed free access to feed, plasma IGF-1 concentrations obtained between 7 and 18 wk of age were positively correlated, both with weight gains prior to sampling, and with total weight gain from weaning to slaughter. Thus, at each age examined, lambs with higher t...